Modern mobile devices continue to demand increasing rates of data exchange. One way to increase the rate of data exchange of a mobile device is by simultaneously transmitting and receiving radio frequency (RF) signals from a single antenna in the mobile device. Although effective at increasing the rate of data exchange that the mobile device is capable of achieving, the simultaneous use of a single antenna for transmitting and receiving signals can result in interference between transmit and receive circuitry in the front end of the mobile device. The interference between the transmit and receive circuitry may be especially problematic for certain combinations of transmit and receive frequencies, such that the mobile device may become unusable in these frequency combinations.
FIG. 1 shows exemplary mobile device front end circuitry 10 for transmitting and receiving RF signals from an antenna 12. The mobile device front end circuitry 10 includes a plurality of low band ports 14, a plurality of high band ports 16, a low band duplexer 18, a high band duplexer 20, low band switching circuitry 22, high band switching circuitry 24, a diplexer 26, and antenna tuning circuitry 28. The plurality of low band ports 14 are coupled to the low band switching circuitry 22 through the low band duplexer 18. Similarly, the plurality of high band ports 16 are coupled to the high band switching circuitry 24 through the high band duplexer 20. Both the low band switching circuitry 22 and the high band switching circuitry 24 are coupled to the diplexer 26, which is in turn coupled to the antenna 12 through the antenna tuning circuitry 28.
In a receive mode of operation, an RF signal is received at the antenna 12, which is passed through the antenna tuning circuitry 28 to the diplexer 26, where it is separated into a low band signal component and a high band signal component. The low band signal component is delivered to the low band switching circuitry 22, where it is then delivered to an appropriate one of the plurality of low band ports 14 through the low band duplexer 18 so that it may be further processed by low band receive circuitry (not shown). The high band signal component is delivered to the high band switching circuitry 24, where it is then delivered to an appropriate one of the plurality of high band ports 16 through the high band duplexer 20 so that it may be further processed by high band receive circuitry (not shown).
In a transmit mode of operation, a transmit signal is provided to an appropriate one of the plurality of low band ports 14 or an appropriate one of the plurality of high band ports 16 from transmit circuitry (not shown). The transmit signal is passed through either the low band switching circuitry 22 or the high band switching circuitry 24 to the diplexer 26, where it is subsequently delivered to the antenna 12 through the antenna tuning circuitry 28. As will be appreciated by those of ordinary skill in the art, the antenna tuning circuitry 28 may include one or more RF switching elements for connecting various components to the antenna 12 in order to alter the impedance presented to the antenna 12. The RF switching elements in the antenna tuning circuitry 28 may be nonlinear, and therefore may generate harmonics of signals passed between the antenna 12 and the diplexer 26.
In certain combinations of transmit and receive frequencies, harmonic components of a transmit signal may fall within the signal band of a receive signal. This may lead to harmonic distortion generated from the transmit signal flowing back through the diplexer 26 and into the receive circuitry. Because the transmit signal is generally a much higher amplitude signal than the receive circuitry is designed to handle, the harmonic distortion may overpower and desensitize the receive circuitry, thereby impeding the performance of the mobile device front end circuitry 10 or rendering it unusable altogether. For example, when transmitting in Band 17 (704-716 MHz), the third harmonic of the transmit signal falls within a Band 4 receive signal (2110-2155 MHz). Accordingly, distortion about the third harmonic of Band 17 generated due to the RF switching components in the antenna tuning circuitry 28 will travel back through the diplexer 26 and into the Band 4 receive circuitry, causing desensitization of the receive circuitry and degrading the performance of the mobile device front end circuitry 10.
FIGS. 2A-2F show conventional RF switching circuitry 30 that may be used in the antenna tuning circuitry 28 of FIG. 1 in a variety of configurations. FIG. 2A shows conventional RF switching circuitry 30 including a plurality of RF switching elements M_RF and adapted to operate in a series configuration, wherein a signal presented at an input node 32 is selectively passed to an output node 34 based on a control signal delivered to a control port 36. FIG. 2B shows conventional RF switching circuitry 30 adapted to operate in a series configuration, wherein the series equivalent of a first tuning capacitor C_TN1 and a second tuning capacitor C_TN2 is selectively presented between an input node 32 and an output node 34 based on a control signal delivered to a control port 36. FIG. 2C shows conventional RF switching circuitry 30 adapted to operate in a series configuration, wherein the series equivalent of a first tuning inductor L— TN1 and a second tuning inductor L_TN2 is selectively presented between an input node 32 and an output node 34 based on a control signal delivered to a control port 36. FIG. 2D shows conventional RF switching circuitry 30 adapted to operate in a shunt configuration, wherein a signal presented at an input node 32 is selectively shorted to ground. FIG. 2E shows conventional RF switching circuitry 30 including a tuning capacitor C_TN and adapted to operate in a shunt configuration, wherein a signal presented at an input node 32 is selectively shorted to ground through the tuning capacitor C_TN. FIG. 2F shows conventional RF switching circuitry 30 including a tuning inductor L_TN and adapted to operate in a shunt configuration, wherein a signal presented at an input node 32 is selectively shorted to ground through the tuning inductor L_TN. As will be appreciated by those of ordinary skill in the art, the antenna tuning circuitry 28 may contain RF switching circuitry 30 in any of the previously mentioned configurations in order to alter the impedance presented to the antenna 12.
As discussed above, while the RF switching circuitry 30 may allow tuning of the impedance presented to the antenna 12 in order to increase the efficiency of the mobile device front end circuitry 10, each one of the RF switching elements M_RF may generate harmonic distortion about a passing signal. The generated harmonic distortion may cause interference between transmit and receive circuitry in the mobile device front end circuitry 10, thereby degrading the performance of the circuitry.
Accordingly, there is a need for RF switching circuitry that is capable of passing RF signals while simultaneously reducing or eliminating the generation of harmonic distortion.